Character States of THAI RUBIACEAE[1]

 

Contents:

 

1. Life forms

     1A. Woody plants

          1A.a. Trees

          1A.b. Treelets (or “monocaul treelets”)

          1A.c. Shrubs

          1A.d. Woody climbers

              Tree and shrub architecture

              Thorns and hooks (hooked thorns).

     1B. Herbaceous plants

     1C. Special life forms

              Tuberous epiphytic ant-plants (myrmecophytes)

              Epiphytes

              Rheophytes

              Geoxylic suffrutices

2. Leaves and Stipules

     2A. Leaves

              Anisophylly

              Bacterial nodules

              Domatia

              Raphides

     2B. Stipules

              Colleters

 
 
 
 
 
 

3. Inflorescences and flowers

     3A. Inflorescences

          3A.a. Terminal inflorescences

              “Leaf-opposed” (or “pseudo-axillary”) inflorescences

          3A.b. Axillary inflorescences

     3B. Flowers

          3B.a. Calyx

          3B.b. Corolla

              Corolla aestivation

          3B.c. Stamens

          3B.d. Gynoecium

              Ovary and placentation

              United ovaries of adjacent flowers

              Nectariferous disk

     3C. Floral biology; pollination and reproductive biology

          3C.a. Protandry and secondary pollen presentation (SPP)

          3C.b. Heterostyly (heterodistyly)

          3C.c. Show bracts and show calyces

4. Fruits, Seeds and their Dispersal

     4A. Solitary fruits

          4A.a. Capsules

          4A.b. Fruits splitting into mericarps

          4A.c. Indehiscent fruits

              Drupes

              Berry-like fruits

     4B. Fused fruits

     4C. Seeds

 

 

1. Life forms

 

1A. Woody plants

The majority of Thai Rubiaceae; either trees, treelets, shrubs or woody climbers.

 

1A.a. Trees. The bulk of Thai rubiaceous trees can be classified as medium-sized or small trees (i.e.,  trees seldom exceeding a height of c. 15 m). In contrast to other primarily woody fami­lies centered in the tropics, Rubiaceae have evolved relatively few large forest trees (i.e., trees 30 m or more tall). Examples of large Thai Rubiaceae trees are few and not often seen: Anthocephalus, if allowed to grow undisturbed, can reach heights of well over 30 m (; the tree is normally conspicuous in open, disturbed, secondary vegetation and then mostly much smaller); Haldina and sometimes also Mitragyna species can reach similar heights. Conspicuously buttressed (and/or ± fluted) boles, in general, are uncommon in Thai Rubiaceae (examples: old individuals of Anthocephalus or Haldina). Scyphiphora, the only Thai rubiaceous mangrove tree, is known to sometimes produce stilt roots.

 

1A.b. Treelets (or “monocaul treelets”).

Definition: very small trees (“miniature trees”) from < 1 m to a few meters tall, with a single main trunk bearing unbranched or branched lateral (plagiotropic) branches.

One of the most common growth forms; especially frequent in good, undisturbed forest; found in numerous genera.

Lateral branches are frequently decussately arranged (corresponding to a plant’s decussate leaf arrangement), i.e., in pairs which are in angles of  90° to each other (e.g. Urophyllum, and many others; ).

In a few genera, only a solitary branch is developed at a main stem node, and the solitary branches follow a spiral (i.e., subsequent branches at always an angle of 90° to the previous) . A very good field character by which Lasianthus (incl. Lithosanthes) and Amaracarpus, for example, can always be recognized immediately (unfortunately branching is often not noted on herbarium labels or in descriptions!). 

 Frequently confused with shrubs, whose architecture, however, differs by having several (rather than a single) trunks (see below). 

 

1A.c. Shrubs.

Definition: A woody plant with several stems arising from its base. [-Canthium sp.]

Shrubby habit is frequently not a good genus character, as genera often contain both shrubs and trees (e.g. Gardenia or Ixora, and many others). In several shrubby taxa, the upper branches start straggling or climbing (the branches are then sometimes beset with hooked thorns as climbing aid; see also further below), or the entire shoot system starts arching over (e. g. Thai Oxyceros). Apart from these climbing, straggling or scandent shrubs, there are also true woody climbers.

 

1A.d. Woody climbers.

Definition: climbers, vines or lianas with distinctly woody main stems.

They reach up into the crown of large forest trees, pro­ducing their inflorescences there (e.g. Coptosapelta). Relatively uncommon in Thai Rubiaceae. Bole climbers (ad­hering themselves to boles of host trees with their adventitious roots), too, are rather infrequent (e.g. Psychotria species and Schradera; the latter usually an epiphytic climber, in growth form not unlike to that of certain Schef­flera species).  

 

Some of the larger genera (e.g. Psychotria or Morinda) may be very diverse with regard to growth/life form and include species that are trees, treelets, shrubs, climbers or even subshrubs (approaching herbs and woody at the base only).

 

Tree and shrub architecture can aid in the identification of certain genera or selected species in the field. Anthocephalus, for example, can be recognized from a distance by its crown made up of ± horizontally spreading, opposite/decussate main branches arranged in tiers, or Catunaregam tomentosa by its ± umbrella-like to flat-topped, wide crown ; Lasianthus and allies are always distinguished and separable from other treelets by invaria­bly having solitary (rather than paired) plagiotropic branches (see also 1A.b, above).

Modular growth of branches (i.e.,  seemingly “normal” branches that are made up of several to numerous short “modules”) may prove to be a very useful identification character.  Such sympodial-monochasial growth is, for example, typical of Aidia and Morinda (in part) or, in a modified form, of Brachytome ().

The presence of long- and short-shoot differentiation, too, can aid in easy identification (e.g. Leptodermis or Catunaregam ). The much abbreviated short shorts often not only bear the bulk of the leaves of a plant but also produce the terminal inflorescences. Due to accessory bud formation, thorns may be associated with leafy/flowering/fruiting short shoots ().

 

Thorns and hooks (hooked thorns). Invariably modified short shoots [i.e. short lateral branches with limited growth; sometimes (potentially) terminated by an inflorescence and hence also interpretable as inflorescence peduncle: Uncaria! ].

The presence of (hooked) thorns is not necessarily a generic character (e.g. Canthium which has species with straight thorns, species with hooked thorns and species lacking thorns). Size, distribution and arrangement of thorns vary: thorns may be several cm long or hardly reach 1 cm in length; thorns may be confined to lateral branches or be present both on the main trunk [ Canthium] and on plagiotropic branches; depending on the leaf arrangement, thorns occur in opposite pairs or in whorls of 3.

Hooked thorns tend to be associated with the climbing, straggling or scandent habit of a plant (obviously serving as climbing aid); their presence, then, is confined to plagiotropic branches (e.g. Uncaria, ). Thai Oxyceros is unusual in having two species groups which differ in the arrangement and size of the hooks: one has relatively small, “simple” paired hooks in all leaf axils of lateral branches [ top] whereas in the other hooks are confined to the basal nodes of modified lateral branches (resulting in a distinctly snake-fang like appearance [ bottom].

 

1B. Herbaceous plants

Quite robust perennials, occasionally somewhat woody at the base (approaching subshrubs; e.g. Knoxia spp.), short-lived perennials, and annuals occur (examples of the latter primarily in the Hedyotis–Oldenlandia complex). Herbaceous growth form in Rubiaceae undoubtedly is a derived condition, and there are often other derived characters that go along with it, such as small to minute, few-merous flowers, etc.

Herbs are either erect (e.g. many taxa of the Hedyotis–Oldenlandia complex), scrambling, straggling or climbing (e.g. Rubia, by means of recurved prickles on stem angles and leaf margins), or prostrate. Prostrate herbs often form carpets or cushions (e.g. Dentella), and

stems not uncommonly produce adventitious roots (e.g. Geophila). There are few examples of herbs producing basal leaf rosettes (some taxa of the Hedyotis–Oldenlandia complex or Knoxia rosettifolia).

Succulence is uncommon but not entirely absent: several Ophiorrhiza spp., for example, have distinctly succulent stems; the coastal Hydro­phylax, growing in the salt-spay zone, is ± succulent in the entire vegetative region.

Tuber-like underground organs are only known from some Argostemma spp. (and possibly also occur in Clarkella), allowing the perennials to survive dry and less humid periods in their specialized habitats; the plants’ aerial parts entirely disappear during dry spells. They may be the only true geophytes in Thai Rubiaceae (A. concinnum: ).

The shoot system of herbs may be extremely reduced. In Clarkella, the vegetative region typically consists only of a (relatively) large solitary leaf above the cotyledons, followed by a pair of smaller, isophyllous leaves (above which is the terminal inflorescence). In some Argostemma spp., there is often only a solitary large leaf (whereas the second leaf of a leaf pair is very reduced and minute); e.g. A. unifolioides .

 

1C. Special life forms

Tuberous epiphytic ant-plants (myrmecophytes), one of the most unusual life forms in Rubiaceae, are represented in the Thai Flora by two genera, both with one species each: Hydnophytum (tuber surface unarmed) and Myrmecodia (tuber surface with spiny protuberances, morphologically representing modified adventitious roots ). In both genera, the tubers morphologically are hypocotyl tubers which start to develop already in seedling stage . The tubers contain a labyrinth of channels and cavities which are inhabited by ants [other types of ant-plants known from the family, i.e. swollen, hollow internodes serving as ant-houses, or modified leaf bases to serve as ant domicile, etc., do not occur in Thailand].

Epiphytes, in general, are rather uncommon in the family. Apart from the ant-plants mentioned above, few Thai taxa fall into this category. One of them is Neohymenopogon which, however, can also be epilithic (growing on rocks), another is Schradera, a climbing epiphytic shrub.

Rheophytes (i.e., plants living in or at the edge of fast flowing streams or rivers which are capable of withstanding seasonal flooding; usually with flexible twigs that do not break easily and narrow “stream-lined” leaves) are relatively more common and are mostly shrubs or dwarf shrubs, less commonly small trees. Examples for obligatory rheophytes: Kailarsenia, Ixora spp., Morindopsis ; for a facultative rheophyte: Hyptianthera.

Geoxylic suffrutices (i.e., subshrubby plants with much reduced vegetative and fertile aerial parts but an extensive underground stem system; primarily occurring in woodland areas with seasonal climate and frequent fires; only the underground parts survive fires, and in the following growing season, new aerial shoots with leaves, flowers and fruits will develop from it) are rare in the Thai flora. The only known Thai Rubiaceae to exhibit this growth form are species of Mussaenda (M. uniflora, previously known as Aphaenandra uniflora), Morinda and Pavetta. Gardenia and Catunaregam, which have also evolved geoxylic suffrutex species, do not show this life form in Thailand (confined to Africa where this growth habit is much more widespread).

 

2. Leaves and Stipules

 

2A. Leaves

Leaves of Rubiaceae are invariably simple, entire and opposite (typically decussate, i.e., in opposite pairs which are at angles of 90° to each other from node to node). In woody taxa, leaf pairs of plagiotropic branches are not uncommonly all in one plane (a lateral branch may thus mimic a large compound leaf; “phyllomorphic branch”; e.g. Lasianthus, ), whereas orthotropic stems bear decussate leaves. Occasionally, leaves are in (true) whorls of 3 (e.g. Wendlandia p.p., ) or even 4 [but also see pseudo-trifoliate, pseudo-ternate and pseudo-whorled leaf arrangement, below!].

In the herbaceous genera Rubia and Galium [tribe Rubieae], the stipules have become leaf-like and, together with the true leaves, are arranged in pseudo-whorls of 4 to ca. 10 [-Galium asperifolium]. In a group of Argostemma species, 2 or more pairs of leaves are always very closely spaced due to highly abbreviated inter­nodes, resulting in a pseudo-whorled arrangement; if this is also combined with anisophylly (see below), the leaves may simulate the presence of a digitately compound leaf (e.g. A. parvum, . Seemingly trifoliate leaves or seemingly 3-whorled leaves in certain woody genera (e.g. Rothmannia, Duperrea, Brachytome) are the result of the reduction of one leaf of a pair and a much shortened internode immediately below the seeming solitary leaf [-Rothmannia].

Anisophylly (i.e., the leaves of a pair slightly to very unequal in size). Relatively widespread in both woody and herbaceous taxa. “Extreme” examples: Mycetia spp. [- M. gracilis], or Argostemma unifolioides; in the latter, the seemingly solitary leaf is the much enlarged leaf of an extremely anisophyllous pair (). The extent of anisophylly often seems to be environment-dependent; availability of light, or lack thereof, might be a main factor for pronounced or indistinct anisophylly. In shade shrubs straggling for light, anisophylly may lead to “phyllomorphic branches” resembling a large digitately compound leaf (Mycetia malayana, ]; also in herbaceous taxa (e.g. Argostemma, ).

In various woody taxa with a differentiation into long shoots and leafy short shoots, the leaves on the two shoot types often differ (slightly) in size and shape. Leaves on orthotropic and plagiotropic shoots may show similar differences.

Bac­terial nodules (= bacterial leaf galls) in the leaf blades (visible as dark spots if a leaf is held against the light). Known from several rubiaceous genera, in Thailand only in Pavetta which can be vegetatively recognized by this feature. Distribution over the leaf blade, size and shape of these bacterial nodules appear to differ between species . Comparable bacterial nodules are present in some Thai species of Ardisia (Myrsinaceae) where they, however, are always confined to the leaf blade margins.

Domatia of various shapes and sizes, and either hairy or not, are present on the leaf blades of numerous woody taxa (normally visible on the lower leaf surface, usually in the axils of secondary veins). As their presence or absence can vary even within taxa, their value as a distinction character is very small.

Raphides [i.e., bundles of crystal needles in specialized cells (idioblasts) which are usually located in the chlorenchyma of a leaf ()]. Their presence is often recognizable under a hand lens, especially in dried leaves, because the needle bundles tend to form minute “pumps” on the leaf sur­faces. Otherwise, crude hand sections are sufficient to see them. – Raphides also occur in other tissues, often in the ovary wall; their presence or absence is of some macrosystematic importance.

 

2B. Stipules

Stipules of all Thai Rubiaceae are interpetiolar, i.e. they arise between the bases of the leaf stalks (petioles). There is a very wide range of forms, sizes and shapes of stipules within the family, but herbaceous alliances show a clear trend towards the division of stipules into several to many segments (fimbriae, bristles). Woody groups often exhibit the opposite trend, i.e. various degrees of stipule fusion: stipules may form a basal sheath but be free above (e.g. Guettarda speciosa: ), or they may become entirely collar-like, sheathing the internode above the insertion point of a leaf pair, e.g. Gaertnera: ).

Stipule characters can aid in the identification of sterile material of a number of genera. Stipules that are bifid at their apex are, for example, typical for Mussaenda or Uncaria (although there exceptions within both genera) . The deeply bifid stipules of Adina distinguish it from other woody genera with globose flowering heads. Ixora is vegetatively readily recognized by mostly having stipules which apically bear a ± long to very long awn-like appendage [- Ixora butterwickii]. Long and narrow (linear to awn-like) stipules are also characteristic for Thai Urophyllum species.

On the other hand, stipules may be of quite diverse shapes and sizes within a genus and, then, are of no diagnostic value at the generic level. An example in place is Psychotria. Similarly, Lasianthus can either have small and narrow or large and broad stipule types. In Wendlandia, some species have ear-like and almost leafy stipules () while others have triangular, cuspidate stipules.

Unfortunately, stipules are falling early in numerous genera and species and are no longer present on older parts of the shoots. The explanation for this lies in the primary function of stipules which is the protection of the shoot apex (after the new shoot has emerged, they are no longer needed). Stipules are, therefore, often most conspicuous on branch tips (e.g. Mitragyna, ).

Colleters (i.e., multi­cellular gland-like structures of diverse morphology and anatomy which excrete a resinous liquid) are typically found on the insides, and sometimes also margins, of stipules. The entire insides (adaxial sides) of stipules can be covered with colleters, colleters can be confined to their basal parts (as in Paederia ) or are found on the stipule margin (e.g. Hymenodictyon ). – Colleters sometimes produce large amounts of resinous excretion, especially in the apical region of a shoot. Anthocephalus, for example, has distinctly sticky apical buds, due to the excretions from the numerous large colleters. In Gardenia, apical buds often have a conspicuous pale yel­lowish to orange droplet of resinous excretion (this, together its stipules which are connate in a cylindrical sheath, make it easy to recognize the genus vegetatively).

Colleters, although normally associated with stipules, sometimes also occur on calyces of flowers (e.g. in a group of  Mycetia species, ).

 

3. Inflorescences and flowers

 

3A. Inflorescences

Inflorescences of Thai Rubiaceae exhibit a bewildering range of variation in extent and size, in shape and arrangement, and in flower number (from very many-flowered to few-flowered and uniflorous). Probably of all of these types can, nevertheless, be derived from a terminal, multi-flowered thyrse to panicle-like inflorescence [-Wendlandia], which is considered the basic inflorescence type in the family. Based on the principle of variable proportions, differing lengths and angles of inflorescences axes, and of peduncles and pedicels, result in a wide range of shapes, from spike- or raceme-like (due to the congestion of lateral partial inflorescence) [-Rennellia elliptica, -Hymenodictyon] to narrowly and broadly pyramidal, to umbel-like [-Morinda umbellata] , or due to length reduction of inflorescence axes, to globose, head-like inflorescences [-Neonauclea pallida].

The basically decussate arrangement of the leaves and, consequently, also that of the lateral axes arising in their axils, is also manifest in the inflorescence and results in very symmetrical sympodial-dichasial cymes. Sometimes, however, branches are produced in the axil of only one leaf (bract) of a pair (sympodial-monochasial branching!), resulting in (partial) inflorescences being arranged in helicoid or scorpioid cymes [-Ophiorrhiza]. The single flower of uniflorous inflorescences [-Kailarsenia lineata] is interpreted as the terminal flower of an inflorescence whose other flowers are no longer developed.

From a practical point of view, terminal and axillary inflorescences can be distinguished (whereby it needs to be noted that plants with terminal inflorescences not infrequently also develop additional axillary inflorescences below the main terminal one). The distinction of the two types is not always easy for the inexperienced: inflorescences borne terminally on very abbreviated shoots which arise in the axils of long shoot leaves superficially look as if they were axillary (e.g. Catunaregam). Another special situation are leaf-opposed (sometimes also called “pseudo-axillary”) inflorescences (see below).

 

3A.a. Terminal inflorescences are most common in the family and found in very numerous Thai Rubiaceae, both woody and herbaceous. Extensively and regularly (dichasially) branched inflorescences with very numerous, although relatively small flowers characterize many genera. In some woody as well as certain herbaceous genera, partial inflorescences switch to monochasial branching (i.e. are made up of  helicoid or scorpioid cymes), e.g. Greenea , Ophiorrhiza, Mouretia, or Spiradiclis. Spike- to raceme-like inflorescences are mostly known from woody taxa (e.g. Hymenodictyon , or Rennellia  in which either sessile or shortly stalked capitula are arranged on the inflorescence main axis ). Inflorescences reduced to few or only a solitary flower are common in, for example, several genera of the woody Gardenieae tribe (e.g. Gardenia or Rothmannia, ); reduction in flower number frequently appears to be compensated by the production of large and showy flowers. Quite a large number of (not necessarily closely related) woody genera possess terminal, globose, head-like inflorescences. Flower heads are either solitary (e.g. Anthocephalus ) or, in turn, grouped together in variously shaped inflorescences (e.g. Mitragyna or Morinda umbellata, ). [The only exception within this group is Haldina which has axillary, pedunculate heads].

There are two fundamentally different types of globose inflorescences: one, in which ovaries of adjacent flowers are free from each other (e.g. Neonauclea, Schradera, etc.), and the other, in which they are fused (e.g. Nauclea or Morinda) (see also further below). In flower, this is not easily seen; a flowering head needs to be sectioned to determine whether ovaries are actually joined by tissue fusion or merely tightly pressed together and separable with a needle or some other suitable fine instrument

So-called “leaf-opposed” (or “pseudo-axillary”) inflorescences are inflorescences which are seemingly opposite a solitary leaf at a node. Upon close examination is becomes apparent that these inflorescences terminate a shoot module, and that the solitary leaf is the bract in whose axil the next shoot module arises (sympodial-monochasial growth) (see ). Examples include Aidia, Brachytome and several Morinda species.

 

3A.b. Axillary inflorescences, too, occur in both woody and herbaceous taxa. In woody groups, they are either present on lateral branches, in the axils of long shoot leaves, or in leaf axils of  the main stems; in some Thai taxa, they are clearly in a supra-axillary position (i.e., arise some distance above a node) (very conspicuous in Morindopsis ). Axillary inflorescences are either (sub)sessile or distinctly pedunculate, consisting of many-, several- to few-flowered cymes; in some woody genera, there are species with either subsessile or with pedunculate axillary flower clusters (e.g. Urophyllum, or Lasianthus: ). Herbaceous groups with axillary inflorescences tend to have dense, subsessile flower clusters in foliage leaf axils (e.g. Spermacoce: , Mitracarpus).

Cauliflory, in general rather uncommon in Rubiaceae, is almost absent in Thai members of the family (one of the few examples is Mycetia cauliflora ).

 

3B. Flowers

The basic type of Rubiaceae flower is a hermaphrodite, radially symmetrical (actinomorphic), 5-merous, sympetalous flower with one whorl of stamens alternating with the corolla lobes, and a presumably 2-carpellate inferior ovary. The calyx, either variously fused or free, is inserted at the top of the ovary, and the roof of the ovary typically produces nectariferous tissue, i.e., a disk (zoophilous flowers! See also 3C).

There is, however, considerable variation, and most of the floral organs can deviate from the basic floral plan: Apart from 5-merous flowers, 4-merous flowers are quite widespread. Merosity may vary within a given taxon, or merosity is fixed within a genus. A good example for the latter are Thai Tarenna (invariably 5-merous), and the allied Pavetta and Ixora (invariably 4-merous flowers). Reduction to 3-merous corollas is mostly restricted to herbaceous groups such as Spermacoce or Richardia. On the other hand, there can also be an increase in floral parts (pleiomery); this is most ob­vious if the increase concerns the corolla: taxa of Gardenia or Catunaregam, for example, can have up to 10 (or sometimes more) corolla lobes [- Catunraregam tomentosa].

Flower sizes vary extremely. Corolla tube lengths range from < 1 mm in some herbaceous groups such as Galium, to well over 10 cm (in woody taxa such as Gardenia), and corolla diameters from ca. 2 mm (e. g. the herbaceous Mitracarpus) to over 4 cm (e.g. woody taxa such as species of Rothmannia).

 

3B.a. Calyx. Differentiated into a basal tubular part ("calyx tube") and free lobes above . Either one or the other may be better developed, i.e. the calyx lobes may be largely reduced while the basal tubular part is conspicuous (e.g. some Gardenia spp. or Rennellia), or the basal tubular part may be poorly developed so that the calyx lobes appear almost free (e.g. Hydrophylax). The calyx may sometimes be almost entirely reduced to an indistinct rim, or to shallow, indistinct teeth, surrounding an annular nectariferous disk (e.g. Prismatomeris - fig. B). In some taxa there are much enlarged (foliaceous), colored calyx lobes (see also 3C.c).

In literature, the terms “calyx tube” and “calyx limb” are frequently used incorrectly and in a misleading way. The former actually refers to the inferior ovary, whereas “calyx limb” is meant to be the entire calyx. This may lead to confusions with regard to measurements. In the present treatment, measurements for inferior ovary and calyx are strictly kept separate [see ].

 

3B.b. Corolla. Always differentiated into a tubular part and the free lobes. Length, width and shape of the tube vary considerably between genera (usually less so within genera), and also the proportion tube : lobe length can show considerable differences. Hypocrateriform (salver-shaped) to infundibular (funnel-shaped) corollas are most common. Corollas with a narrowly cylindrical base widening into a distinctly bell-shaped portion above are uncommon (e.g. Rothmannia witii, ), as are urcelliform corollas (basal part of tube widened; e.g. Mycetia: and Ophiorrhiza spp.). Considered derived are flowers with a largely reduced corolla tube [e.g. (sub)rotate corollas in Galium or Rubia ].

The most common corolla colors are white, creamy-white, greenish-white, yellowish-white, yellowish-green and pinkish-white to pink. (Intensively) red and yellow flowers are relatively uncommon in indigenous Thai taxa but frequently seen in cultivated genera such as Ixora. Intensively blue flowers are, in general, also rather rare in the family and not seen in Thai representatives. In several genera, there is a distinct age-dependent color change. The most conspicuous example is Gardenia, with white or creamy-white flowers turning yellow to orange with age (); also the reverse situation (yellow flowers becoming pale yellow to whitish with age; e.g. Mycetia spp. ). In only a few Thai genera, the throat is colored strikingly different from the rest of the corolla (e.g. dark red to maroon in Paederia , or orange to orange-red in Mussaenda spp.).

In few alliances, the corolla tubes are distinctly curved, making the flowers slightly zygomorphic [e.g. Chassalia, or Psychotria (Cephaelis group, in part) ]. Much rarer are unequally sized corolla lobes (making the corolla slightly bilabiate; e.g. Mycetia chasalioides )

Corolla aestivation (arrangement of the corolla lobes in bud stage) varies within Rubiaceae. The most common situation is valvate aestivation, where the edges of corolla lobes merely touch each other. Two modifications of this aestivation type are valvate-induplicate where lateral parts of adjacent corolla lobes are folded inward in bud (the originally infolded parts are often recognizable in open flowers as thin lateral flanges of corolla lobes; e.g. Saprosma; ) and valvate-reduplicate where lateral parts of adjacent corolla lobes are folded outward (; buds then often distinctly ridged; conspicuous, for example, in several Mussaenda species). Also rather frequent is contorted aestivation, where corolla lobes are distinctly twisted (mostly to the right) in bud stage []; even open flowers often still show this (e.g. Gardenia). A third kind, imbricate aestivation (lobes variously overlapping) is more uncommon in Thai Rubiaceae (e.g. Anthocephalus, Luculia).

 

3B.c. Stamens. In Thai (and most other) Rubiaceae always present in the same num­ber as the petals; always differenti­ated into filaments and anthers. The latter vary in shape, size and insertion point of the filaments (dorsi-, medi-, basifxied). Stamens (anthers) either (a) remain included in the corolla tube, (b) barely exceed the level of the throat or (c) are long ex­serted (see also 3C.b). Stamens (an­thers) are typically free, nota­ble exceptions amongst Thai Rubiaceae are the genera Argostemma and Acranthera in which either anthers are truly fused (synandry) or at least in a cone-like arrangement [, - Argostemma] (linked a special pollination mode, i.e. “buzz-pollination”).

 

3B.d. Gynoecium. The basic bicarpellate gynoecium is most frequently met in Thai Rubiaceae, but some genera/species show a larger number of carpels (up to a maximum of c. 9): occasional increase from 2 to 3 (e.g. Canthium, sometimes within species, or Wendlandia); 3 to 4 carpels (e.g. Richardia); 5 to 4 carpels (e.g. Meyna); 2 to 9 carpels (Gardenia); 3 to ca. 9 carpels (e.g. Lasianthus: , Guettarda). In one genus (Timonius), the number of carpels is secondarily increased (to over 100, although not as many in Thai taxa).

The gynoecium is differentiated into ovary, style and stigmas. The style is simple, varying considerably in length. The stigmas, in number, correspond to the number of carpels present; shapes and size are highly variable.

Ovary and placentation. The carpel number mostly corresponds to the number of ovary locules because septa are typi­cally present (see further below for exceptions). Each locule can contain very many, many, several, few or only a solitary ovule (). Ovule number can, to a certain extent, aid in identification of alliances (e.g. members of the tribes Spermacoceae and Hedyotideae, mostly herbs which may closely resemble each other in habit, are easily distinguishable by either having always only 1 ovule or, in the latter, several or, more commonly, many ovules per locule). There are, however, also docu­mented reductions series within certain groups (e.g. several, few or only 1 ovule per locule in Tarenna).

In most Rubiaceae, the ovules are attached to an axile placenta. Placenta sizes and shapes vary greatly, as does their point of attachment (along the entire length of the septum; around the middle, near the top or near the bottom, etc.). Certain groups are distinguished by solitary ovules either attached near the base of the septum (“basal erect ovule”; e.g. Psychotria and allied genera, , or Paederia), or near the apex of the locule (“apical pendulous ovule”; e.g. Canthium, Knoxia, or Guettarda). In genera of the Gardenieae (Gardenia, etc.), placentas are often proliferous, i.e. they start growing around the ovules, eventually immersing them; in fruit, the placentas become a fleshy pulp in which the seeds are totally embedded.

The reduction of septa leads to unilocular ovaries (and parietal placentation). This is uncommon in the family and, in Thailand almost entirely confined to Gardenieae genera (Gardenia, etc.). Another unusual situation is the occasional development of false septa or “pseudo-septa” (i.e., outgrowths of the carpel wall extending toward the centre of an ovary, thus resembling true septas). Due to their presence, the ovaries of Urophyllum, for example, are seemingly multiloculate. The ovaries of Morinda are actually bicarpellate (with 2 ovules per carpel) but appear to have 4-locules with 1 ovule each for the same reason.

Gaertnera is the only Thai rubiaceous genus with a secondarily superior ovary, although trends to (secondarily) semi-superior ovaries are noticeable in some herbaceous members of the Hedyotideae (Hedyotis, Oldenlandia), which then may show a close resemblance to herbaceous Loganiaceae such as Mitrasacme.

United ovaries of adjacent flowers. In several alliances, several (e.g. often 3–5 in Rennellia) to rather many (e.g. Morinda or Nauclea) ovaries of adjacent flowers are connate due to true tissue fusion. In such cases, flowers are arranged in head-like inflorescences (“capitula”) which may be stalked and solitary (e.g. Morinda p.p.) or grouped together into a more extensive inflorescences (i.e. umbel-like as in Morinda spp., , or raceme- to spike-like, as in Rennellia ). The Thai species of Mouretia is exceptional in that the ovaries of all flowers of the heli­coid/scorpioid cymes making up an inflorescence are united ().

 

Nectariferous disks. Typically present (because the predominantly zoophilous rubiaceous flowers offer nectar as reward to visiting pollinators; see also 3C) and located on the roof of the inferior ovary. Most Thai Rubiaceae have ring-like disks surrounding the base of the style (-top). In numerous taxa they are large and persistent, remaining visible after the corollas have fallen (e.g. Mouretia, ). Differently shaped disks, however, also do occur (e.g. distinctly 2-lobed in Galium and Rubia). Nectariferous disks are lacking only in those few taxa which offer pollen rather than nectar to visiting pollinators; this is the case in buzz-pollinated groups (in Thailand probably Argostemma and Acranthera only). Also genera that secondarily have switched to wind-pollination do not develop them (in Thailand only Nertera).

 

3C. Floral biology; pollination and reproductive biology

Flowers of Thai Rubiaceae are almost exclusively zoophilous (i.e., pollinated by animals). The wide range of flower forms, sizes and colors suggests that many different pollinators are involved. Most of these are various insects while birds and bats play no important role. The bulk of Thai Rubiaceae flowers is probably pollinated by various bees (relatively small flowers), by day-active butterflies and by night active hawk-moths (mostly relatively large and showy flowers, scentless in the former but fragrant and often longer-tubed in the latter).

The flowers are predominantly hermaphrodite but in several alliances flowers have become (secondarily) unisexual (e.g. Brachytome, Ceriscoides, Timonius or Urophyllum). In Thai unisexual repre­sentatives, male and female flowers are distributed on separate individuals, i.e. the plants are dioecious. Unisexuality is not always obvious at first sight because well discernable rudiments of the opposite sex are often present. Dioecious taxa frequently show a sexual dimorphism which expresses itself by differences in the extent of the inflorescences (female inflorescences tend to be fewer-flowered than in males; e.g. Timonius ), and/or by different flower sizes.

 

3C.a. Protandry and secondary pollen presentation (SPP). In most Thai rubiaceous flowers,  the anthers dehisce and release ripe pollen before the female parts are fully developed and become receptive, i.e. protandry prevails.

In many alliances, protandry is combined with SPP, a phenomenon also known from various other plant families. SPP, in general, refers to situations where the pollen of a flower is presented to a pollinator by organs other than the dehisced anthers themselves [The latter situation is “primary” presentation of the pollen]. In Rubiaceae exhibiting SSP, it is normally  the upper part of the style and/or the sterile outsides of the stigmatic area which serve as “pollen presenters;” as, in Rubiaceae, this phenomenon was first noted in Ixora, it is sometimes also called “ixoroid pollination mechanism.”  In principle, it works as follows: In late bud stage, anthers shed their pollen onto the stylar or sterile stigmatic areas which are usually provided with certain adaptations that make the pollen stick [-fig. a]. When the flower opens, the style straightens out and/or elongates, thus often lifting the pollen high above the level of the corolla [-fig. b]; at this stage the stigmas are not yet receptive. Pollinators will now remove the pollen from the style/stigma areas. After a period of time (in some cases several days), the flowers move on to their female phase; the stigma lobes unfold and become receptive [-fig. c]. This transition from a functionally male to a functionally female stage over a period of time drastically reduces the chance of a flower’s own pollen germinating on its own stigma. Some Thai examples: Ixora, Pavetta , Duperrea or Catunaregam; also  many of the Rubiaceae with showy flowering heads (Anthocephalus, Mitragyna, Uncaria, etc.).

 

3C.b. Heterostyly [or, more precisely, heterodistyly]. Refers to two types of flowers within a species (population) differing in anther and stigma position: Long-styled morphs with elongated styles which lift the stigmas above the level of the anthers and short-styled morphs with shortened styles (so that the stigmas are situated below the level of the anthers) . In both morphs, anthers and stigmas thus are spatially separated. Statistically, the distribution of long- and short-styled morphs is about 50 : 50 % within a population. Heterostyly has often been said to be linked with self-incom­patibility, but actual experimental proof is lacking for all but very few of the tropical Rubiaceae (e.g. Luculia; ). In the Rubiaceae there are more heterostylous species than the heterostyles from all other plant families combined.

Heterostyly is not always as obvious as in the diagram and illustrations: sometimes both stigmas and anthers, although at different levels, remain included in the corolla tube (“cryptic” heterostyly). A corolla must be opened to see it (e.g. species of Ophiorrhiza).

Heterostyly is more common in herbaceous alliances, although there are also several woody Rubiaceae showing the phenomenon (e.g. Morinda, Chassalia, Psychotria). Many heterostyles are found in the Oldenlandia–Hedyotis complex. Heterostyly is often not a good generic character but useful in distinguishing species. Within Saprosma, for example, there are both hetero- and isostylous species. Within derived herbaceous genera, there may be a “break-down” of heterostyly, i.e. transitions from heterostyly to isostyly and to autogamy, and even to cryptogamous flowers (e.g. Ophiorrhiza).

It must be noted that heterostyly and SPP never occur within one and the same alliance. By comparing Plate and it becomes obvious why this is so: For SPP to work, anthers and style/stigma portions serving as pollen receptors must be at the same level in bud stage; in heterostylous taxa anthers and stigmas as always spatially separated so there is never a chance to deposit pollen on a flower’s own stigma.

 

3C.c. Show bracts and show calyces. Either enlarged, colored leaves (bracts) subtending (partial) inflorescences (and thus are not part of flowers!) [-left], or (at least some) calyx lobes of a flower that are enlarged, ± leaf-like and colored (“petaloid”) [-right]. Both add to the optical attractiveness of an inflorescence and  thus aid in attracting pollinators. The Rubiaceae is one of the few families showing both phenomena. Thai examples: Neohymenopogon (show bracts; [-left]); Mussaenda, Pseudomussaenda (show calyces; [-right]).

 

4. Fruits, Seeds and their Dispersal

 

4A. Solitary fruits

Dividable into those that (4A.a) split open at maturity, (4B.b) break into one-seeded mericarps (“partial fruits”), or (4B.c) remain indehiscent.

 

4A.a. Capsules. Fruits with a dry and often ± hard wall, splitting open loculi- or septicidally (or, in a few cases, both loculi- and septicidally). A very common fruit type in Thai Rubiaceae. Examples: Hymenodictyon, capsules dehiscing loculicidally into 2 valves; Luculia, capsules dehiscing septicidally, although at length also starting to open loculicidally in the upper part of the fruit []. More “specialized” capsules (i.e., those in which the upper part of the capsule comes off like a lid; “lid capsules”) are uncommon (e.g. Argostemma or Mitracarpus ). 

DISPERSAL. Because capsules split open, they expose and release the seeds (which are thus the dispersal unit). Anemochory (dispersal by wind) is undoubtedly very widespread; especially taxa with winged seeds (- Mitragyna; see also 4C) are well adapted. Other kinds of dispersal modes are recorded but uncommon. Notable is ombrochory, i.e., dispersal by water drops, in herbaceous taxa with erect, cup-like capsules (Argostemma, Ophiorrhiza; rain or run-off water drops falling into them ejaculating the seeds from the “fruit cups” ).

 

4A.b. Fruits splitting into mericarps. In general, rare in Rubiaceae, in Thailand only in Galium.

 

4A.c. Indehiscent fruits.

Drupes. Fruits with a skin-like outer layer (exocarp), a thick, fleshy and often watery tissue layer (mesocarp) and a hard sclerenchymatic innermost layer of varying thickness (endocarp). Probably the most common fruit type in Thai Rubiaceae occurring in many, not necessarily related woody alliances and also in some herbaceous groups. The innermost endocarp layer forms a pyrene or “stone” which encloses a single seed or, less commonly a few, several or many seeds. Sclerenchymatic endocarps of several carpels can fuse, giving rise to a plurilocular stone (e.g. Guettarda, ,).

A very common phenomenon is the often dramatic development-dependent color changes in drupes [-Psychotria]: from immature to fully ripe, drupes of one and the same Lasianthus species, for example, change from whitish to pinkish, then to blue-violet and finally to dark blue. In Psychotria spp. fruit color changes greenish-yellow à yellowish à yellowish-orange à orange-red à red à dark-red-blackish à shiny black are not uncommon [fruit colors given on herbarium labels, therefore, are often highly misleading (the state of development of the fruit should be noted in addition to fruit color, e.g. “rather immature fruit yellowish”)].

Berry-like fruits. “True” berries (defined as having a thick, fleshy fruit wall without distinct, hard endocarp), are generally quite uncommon in Rubiaceae. Many rubiaceous fruits described in literature as “berries” are, in fact, (modified) drupes as they do have a sclerenchymatic endocarp, although this may be rather thin. The term “berry-like” is used for these (e.g.  Mycetia; fruits with a spongy, watery mesocarp, inside of which there is a distinct, although thin and horn-like endocarp coating the interior of the locules ).

DISPERSAL. Ornithochory, i.e. dispersal by birds, is the prevailing dispersal mode of drupes and berry-like fruits. For this reason, they typically have showy colors (white [-Mycetia], yellow [-Canthium], orange, orange-red [-Aidia], red, blue [-Lasianthus], dark shiny purple, shiny black). Contrast colors are known to enhance the attractiveness to birds; contrasting colors of fruits and inflorescence axes of Ixora spp. and Chassalia, need to be mentioned in this context (bright red or white peduncles and pedicels and shiny black mature drupes; in Chassalia, moreover, swollen and ± succulent axes; ). The large fruits of various members of the tribe Gardenieae, being a lesser modification of drupes in that their mesocarp is quite fibrous and hard (rather than soft, spongy and watery), are often less showy and most likely eaten by various mammals; in a few Gardenia species they are known break open irregularly to expose a red fleshy pulp (= proliferating placentas) in which the seeds are embedded (switch to ornithochory!). Drupes can also be modified and adapted to dispersal by sea currents [rather than being fleshy, the mesocarp is fibrous or spongy and air-filled (making the fruits buoyant)]: Guettarda speciosa, Scyphiphora [] [in Thai Rubiaceae, hydrochorous dispersal units may, however, also be of different origin: e.g.  Hydrophylax () whose indehiscent, floating fruits are derivable from capsules which characterize its closest relatives. See also Morinda citrifolia under 4B].

 

4B. Fused fruits

In those taxa where adjacent ovaries of flowers of a partial (or entire) inflorescence were united by tissue fusion before and at anthesis (see also 3B.d), the (few to many) fruits, too, are fused and form a common dispersal unit. The entire structure is called either an “infructescence” or a “syncarp.” Examples are Morinda or Rennellia. In Thai taxa of the tribe Naucleeae in the wide sense, all with globose flowering heads, ovary fusions and, subsequently “infructescences” or “syncarps” are only developed in Nauclea () [in others, fruiting heads consist of many free fruits which may, however, be tightly pressed against each other. Care must be taken to evaluate this character correctly].

DISPERSAL. The predominantly distinctly fleshy fused fruits are mostly zoochorous (an exception being Morinda citrifolia which, under natural conditions, grow in coastal areas and is dispersed by sea water currents; its fruits float because the pyrenes contained therein have air-filled bladders).

 

4C. Seeds

Rubiaceous seeds are “exotestal”, meaning that they have a seed coat consisting of an exotesta only [the exotesta corresponds to the integu­ment epidermis of an ovule in flowering stage. The integument cells layers below the epidermis develop into a seed’s endotesta but in rubiaceous seeds this normally becomes crushed during maturation and is, therefore, usually not clearly discernible]. In numerous taxa, the walls of the exotesta cells have local secondary thickenings which are highly variable in their micromorphology (and sometimes of considerable systematic relevance). As the structure of these thickenings can often not be interpreted easily without elaborate preparation and the use of a Scanning Electron Microscope and microtome techniques, they are of limited usefulness from a practical (flora) point of view and will not be further discussed here.

Seeds show much variation in sizes and shapes (apart from the presence or absence of wings; see below). Many-seeded fruits tend to have small, often variously angled or wedge-shaped seeds [- Mycetia]. Seeds contained in pyrenes usually fill out the locule completely and tightly press against the hard endocarp; their exostestas tend to be rather thin and poorly developed. The normally big, drupe-like fruits of Gardenieae frequently have several to many relatively large, often compressed and lentil-like seeds [not uncommonly embedded in a fleshly pulp (=proliferating placenta)].

 Winged seeds. The variously shaped and sized wings invariably are exotesta outgrowths or extensions; the centrally located, embryo-containing part of such seeds is usually compressed and flattened. In Thai Rubiaceae winged seeds are mostly bi- or tripolar, i.e., the wings are comprised of a long apical wing portion plus two short wings at the base, around the hilum (- the arrow points to the hilum). Many of the genera with globose, head-like inflorescences which produce capsular fruits have winged seeds (e.g. Haldina, Mitragyna , Uncaria).

Rubiaceous seeds always contain endosperm. Typically, the endosperm is copious, but in some alliances it is reduced to few cells layers. As a rule, seeds with extensive endosperm have relatively small (to minute) embryos (e.g. Coffea; the “coffee bean” consists almost entirely of endosperm), while those with comparatively little endosperm have relatively large embryos.

 

 



[1] C. Puff